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MG MGB Technical - engine rebuild
|ive just had my b on the dyno.the spec is peter burgess fast road head,285 cam,bored +60, new shells ,new aldon dizzy and electron ignition .the engine produced just over 100 bhp at the wheels .i thought i may of got closer to 120 bhp .do you think may be i lost alot of power by just fitting standard pistons. the engine revs well and every thing is balanced .thanks daz|
|Daz. Do you have a copy of Peter's book, "|
How to Power Tune MGB 4-Cylinder Engines"?
Peter does not mention the Piper 285 cam, but does mention the Piper 270 Cam. About 95 HP with the twin 1.5" SUs and about 98 HP with the twin 1.75" SUs. It would seems that your slightly "hotter" cam is right within the ball park of what could be expected of the system you put together.
Bottom line would be how does it run? With almost 50% more HP at the wheels than a standard engine, I would think it runs quite well.
No, slightly over 100 HP at the rear wheels doesn't seem to be enough power output for the modfications that you've made.
What modifications have you done to the exhaust system? If it's stock, then the engine can't efficiently exhale the extra ehaust gases produced, which in turn will effect its ability to inhale. For what you describe, either the Maniflow LCB system or the Peco Grey system should be a good choice.
do not underestimate the exhaust system, as Steve allready stated.
With the 285 Piper cam and normal (dished) pistons the effectfe compression ratio will drop to about 7 instead of the 8.8 geometrical calculated cr.
Flat top pistons and a skimmed head will rebuilt this and add some HP.
For details just have a look at the Abingdon Special Tuning Manual and read the details up to stage 6.
With a higher rate of compression and running on SU HIF 6 carbs you will be able to reach ~112 to 115 rwhp with the cam you are running.
|i have a standard bore ss exhaust. one silencer one bullet box and a tubular manifold,the carb as had a service kit fitted new butterflys etc and a K&N large airfilter(the one which retains the orig base plate).the pistons and con rods were drilled so they are all same weight.i had a small problem as the car was running very rich with the AAA needles so i had different needles fitted . the car runs well ,not a lot at the low end but it does give you a kick at 2500 revs and theres a large growling noise fron the air filter. can you explain about the comp ratio as th head was skimmed and ported and explain how some of the race engines are 160 bhp .should i be happy with what ive got . may be you forget the difference between my engine and a standard engine .|
Ignore Steve S - you won't get over 100 from those mods - in fact I reckon your figures are over optimistic and are probably closer to the low to mid 90s
|Chris at Octarine Services|
The 95 is a blueprinted std race and the 98 is a 1.9 litre
As above, my fast road engines with a P270 produce 87 bhp at wheels on Peter's RR so I reckon the P285 might lift this another 4/6 bhp
|Chris at Octarine Services|
the robbing of effective cr is an effect caused by the longer duration of 'hotter' cams.
The cam you use is perfect for road (if set exactely with dial gauges) use but it looses some cr due to the longer overlap of the valves. For some more efficancy the cr can be altered by milling the head within close limits and use flat top pistons. You will need 100(+) Octane fuel with this kind of mods.
Doug Jackson did some reserch and mesurements upon cam duration and effective cr.
You will find more details explaned on his homepage www.mgbmga.com
|may be the problem is i was driving my other car tvr chimaera 4l last summer and when i jumped in to the mg it felt so slow .i will never get rid of my mg as ive spent 8k on it in five years and it as a lovely feel when i drive it daz|
With a set of standard 8.8:1 compression ratio pistons and an +.060 bore (1868cc displacement), a clearance depth of .040", 43cc Combustion Chambers, and an Original Equipment specification camshaft, the Geometric Compression Ratio increases to 9.1:1, and the Effective Compression Ratio increases from 6.7:1 to 6.9:1. With a Piper BP285 Camshaft the Effective Compression Ratio increases from 6.0:1 to 6.2:1. So, the difference between an Original Equipment specification camshaft and the Piper BP 285 is a drop of Effective Compression Ratio of about .5:1.
The power output figures in Peter's book all date to 1996 or earlier. He's made progress since then. Piper claims that their BP285 camshaft will produce a 18HP boost over the Original Equipment camshaft, 6 more than the Piper BP270 camshaft, so your guesstimate seems reasonable. Of course, they make no mention as to whether or not their figures hold true for a stock head, or for a flowed head. Like Ralph, I believe that the engine that Daz describes does have the potential to break the 100HP barrier, although Ralph's 112-115HP is somewhat optimistic. Perhaps fiddling with the ignition curve and a future boost in compression ratio would turn the trick? Of course, there's always that old interpretation bugaboo as to Horsepower and Brake Horsepower.
The 2,500 RPM power surge is about right for a Piper BP285 camshaft. Do you have an Email Address?
Oh, I forgot to mention: as Ralph sugggested, a pair of 1 3/4" SU's and a Maniflow intake manifold should do it. Get out the checkbook!
|DAZ- I have the exact spec on the 78 roadster: Burgess FRBV head, BP 285 which idles at 750 without lump,new hardened lifters, new 1 3/4 SU from Berlen with BAF as I recall,60 over, 9.5 estimated CR, GB123 dizzy, Peco out, one box. On the dyno I achieved a 123 RWHP ( 134# torque) on the Moss SC engine, I could only muster a 84 RWHP, tipping over at 5800, whereas the SC topped at 5100RPM, suggesting both could benefit from a looser exhaust. FWIW VEM |
PS_ Let me qualify "new"= build is 2 years old!
Your engine went ok at 100 at the wheels on my rollers, it was an engine you sorted out with one of our heads and an HR285 cam.
The bhp was ok and I would expect only 6 or so more bhp from one of our engines. To get 120 at the wheels with your cam you need 11.8:1 CR and a good comp type exhaust system and race pistons with thin rings.
To get 150 at the wheel you need a big weber with at least 40 mm chokes. Full Race engine at 1950 with 11.8:1 CR, Full Race cam and a power band from 4250 to 7000 rpm.
|With one of Peter's FRBV head, Peco manifold and exhaust, electronic ignition, HS6s and a 717 cam I got 90bhp.|
I've 'just' (it's taken me a year) fitted a 285 cam and upped the CR from 9.5 to 10.5 on Peter's advice. The car is noticeably quicker. Much more responsive too. I've just fitted a new set of HS6s (the old one were ratty), next job is a rolling road session.
VEM - you must be running a 2 tonne flywheel to get that cam to idle smoothly at 750rpm! It's a bit diifcult to say too much about the 285 because my engine needs tuning, but the 717 made the exhaust sound like a big single cylinder motorbike - even at 1000rpm...
Peter - can you recommend anyone down here (south of Bristol) for a rolling road job? It's a 3.5hr run to your place even in a 'normal' car!
|so as peter said its 100 bhp and im happy ,keeps up with cars today and is a bit of fun... i should of payed the extra and gone for the better race pistons and had it re-lined, i surpose every thing you do adds a little extra bhp.i have a standard bore exhaust and tubular manifold as it as only one silencer and a bullet tube as i just wanted, a nice tone .would the large bore exhaust make any difference ? . how do you get the higher cr|
|forgot to say the best thing i ever did , purchase a new dizzy ..aldon vacume advance with electronic ignition .this mod completly changed the car|
|Neil- It's a lightened fly wheel. Maybe I can record the idle and put the sound on an "Upload" file . Both the Peco ( on the 78 285) and the CI stock LCB exhaust( on the SC Moss) are slated for extrudihone with intent to remove up to 60 though on the inside walls. VEM|
Sorry I don't know of any dyno's I can recommend.
This thread was discussed between 17/02/2008 and 24/02/2008
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